1. Field of the Invention
This invention relates to a method of producing tough steel plate that is homogeneous and nonoriented and more particularly to a method for producing such steel plate without reheating following casting, wherein the desired steel plate is obtained simply by casting or by rolling at a low reduction ratio after casting.
2. Description of the Prior Art
In the production of steel plate by hot rolling there has in recent years been a strong movement toward the realization of cost reduction through the simplification and elimination of production steps. The hot charge rolling method in which the intermediate step of reheating between casting and hot rolling is eliminated is one example of this trend.
However, most of the hot charge rolling processes developed to date rely on an ensuing rolling step for the production of fine crystal grains and have not been able to realize the elimination or simplification of the rolling step.
In this specification, the term "hot charge rolling" will be used to mean a process in which a slab is subjected to hot rolling following casting or continuous casting.
While it is well known to be metallurgically feasible to conduct hot charge rolling of steel plate with respect to mild and ordinary steels for which the low-temperature toughness requirements are not so stringent, application of the hot charge rolling process to, for example, low-temperature steels and high-tensile steels requiring low-temperature toughness results in products with low-temperature toughness that is inferior to those obtained by conventional production processes.
The main reason for this is that in the hot charge rolling process the initial austenite grains present prior to rolling are extremely large, making it difficult to realize a fine-grained texture through rolling. As a way of avoiding this problem it has been proposed to conduct strong controlled rolling prior to controlled cooling. (See, for example, Japanese Published Unexamined Patent Application No. 57(1982)-131,320.) However, the use of this method introduces an additional requirement for time-temperature control and thus greatly impairs the productivity of the rolling process.
For overcoming the limitation on the fineness of the texture obtainable by rolling there have been proposed methods such as that disclosed in Japanese Published Unexamined Patent Application No. 61(1986)-213,322, which relates to a method wherein oxide-system inclusions including a composite crystalline phase consisting of either or both of TiO and Ti.sub.2 O.sub.3 are used as transformation nuclei. With this method, however, the quality of the steel is greatly affected by ther amount of Ti incorporated and, moreover, precise control of the amount of Ti in the refinement step becomes difficult.
As regards methods which use Ti-system inclusions as transformation nuclei for realizing a fine-grained texture, there are known techniques for attaining high toughness of welded metal or of the heat-affected zone (HAZ) in welding.
In the production of steel plate, for utilization of Ti oxide-system precipitates as transformation nuclei it is necessary for the Ti oxide precipitates to be fine and well dispersed. If excessive Ti is added, the residual Ti in solid solution will lead to precipitation harding and this in turn will impair toughness, particularly at welding heat affected zones and the like. Precise control of Ti content must therefore be carried out at the refining step. As this is not easy, it is difficult to realize stable and efficient production.